lxd/instance/drivers/qemu: Drop string builder arg
lxd/instance/drivers/qemu: Drop string builder arg

This drops the string builder argument from

    * addRootDriveConfig()
    * addDriveConfig()

The argument is obsolete as drives are now added using QMP.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd